The N54 and N55 engines make some noise. Mostly it is due to the direct injection system. It's under so high pressure that it kind of makes a diesel rattling sound - when working. This is NORMAL. There have been numerous injector updates over the years. Its best to also make sure your car has the latest ones.
Some early N54 engines had problems with the wastegate actuator rod wearing. Those units has a 8mm rod, while later units had a 10mm rod. ALL 135i's got the later (10mm)design. But still the WG's would rattle under steady throttle conditions. BMW will replace your turbos for this condition.
Of course you could have something simply as a loose heat shield under the car. Also the vacuum hoses(on N54 engines) seem to pop off, the WG actuators.
One easy test is to either remove or "pinch" the two vacuum lines going to these actuators to see IF your WG's are causing the noise.
normally a 30ff code means your turbos are bad though. But its better to check these vacuum hoses first!
